1. Field of the Invention
The invention relates to a coil for transmitting and receiving radio frequency (rf) electromagnetic waves for performing a nuclear magnetic resonance (NMR) study of an object. More specifically, the invention relates to a coil which is capable of transmitting or receiving high frequency rf waves and is tunable across a range of high frequencies.
2. Description of the Prior Art
In gathering NMR information about a sample, rf coils placed around the object being studied commonly perform two functions. The coils transmit rf signals into the object in order to excite its atoms, and as those atoms return to their ground state, they emit rf signals which the coils detect. The functions of transmitting and receiving may be performed by a single coil or by separate coils. In addition, in multi-nuclear studies in which the atoms of each of several elements are excited by rf waves and emit rf waves at a characteristic frequency, more than one coil may be used or a single coil may be tuned to multiple frequencies.
The two types of coils which have traditionally been used for NMR studies of an object are the simple solenoid and the saddle coil. Which coil is used will typically depend on the geometry of the structure which provides the main static magnetic field. For example, if the geometry only permits the object under study to be inserted in a direction perpendicular to the lines of flux of the main magnetic field, a solenodial coil is most efficient. This is the case when the main magnetic field is provided by a resistive or a permanent magnet. On the other hand, when the object under study is inserted in the same direction as the lines of flux, as in a superconducting magnet which has a central bore, the saddle coil must be used. In general, however, the saddle coil may be three times less efficient than the solenoid, as discussed by D. I. Hoult and R. E. Richards, "The Signal-to-Noise Ratio of the Nuclear Magnetic Resonance Experiment", Journal of Magnetic Resonance, Vol. 24, 1976, pp. 71-85.
A fundamental limitation on the use of the simple solenoid and saddle coils is that each becomes extremely inefficient when the length of the coil is comparable to the wavelength of its resonant frequency. Therefore, for high frequency rf signals, whose wavelengths are accordingly relatively short, the phase shift of the rf wave as it passes along the coil reduces efficiency. Furthermore, control over the tuning of the coil may be lost at high frequencies because the distributed capacitance of the coil wires becomes increasingly significant and results in a self resonant condition at a sufficiently high frequency. These effects require a trade-off between frequency and coil size--as the rf frequency becomes higher, the size of the coil must become smaller.
A number of coil designs have been proposed to overcome these limitations by providing a high frequency rf coil of greater size than would be possible with the simple solenoid or saddle coil. One proposed solution is to provide solenoidal and saddle-shaped coils having reduced inductance, which may therefore be tuned to higher frequencies. This design still requires a compromise between frequency and size and results in a less homogeneous magnetic field.
Several coil designs have been proposed based on the properties of an rf transmission line. H. J. Schneider and P. Dullenkopf, "Slotted Tube Resonator; A New NMR Probe Head at High Observing Frequencies", Review of Scientific Instruments, Vol. 48, No. 1, January 1977, pp. 68-73, disclosed a slotted tube resonator (STR) based upon the behavior of rf transmission lines. Variations of the STR are disclosed by D. W. Alderman and D. M. Grant, "An Efficient Decoupler Coil Design which Reduces Heating in Conductive Samples in Superconducting Spectrometers", Journal of Magnetic Resonance, Vol. 36, 1979, pp. 447-451 and by A. Leroy-Willig, L. Darrasse, J. Taquin and M. Sauzade, "The Slotted Cylinder, An Inductive Structure for NMR Imaging", The Society of Magnetic Resonance in Medicine, Second Annual Meeting, Aug. 16-19, 1983, San Francisco, pp. 213-214. I. J. Lowe and M. Engelsberg, "A Fast Recovery Pulse Nuclear Magnetic Resonance Sample Probe Using a Delay Line", Vol. 45, No. 5, May 1974, pp, 631-639 disclosed a lumped parameter delay line which behaves as a transmission line, a design which was modified in I. J. Lowe and D. W. Whitson, "Homogeneous RF Field Delay Line Probe for Pulsed Nuclear Magnetic Resonance", Review of Scientific Instruments, Vol. 48, No. 3, March 1977, pp. 268-274. Published European Patent Application No. 0 047 065 disclosed a distributed phase rf coil using a transmission line with alternately shielded and unshielded sections. By appropriate coupling, the coil may be driven in quadrature to generate a circularly polarized rf wave.
In general, the coil designs based on rf transmission lines are relatively complicated structures with limited or difficult tuning. The STR is especially bulky in relation to the object being studied, and includes solid materials which interfere with the switching of gradients as is necessary in NMR imaging or flow and diffusion measurement. The lumped parameter delay line is generally inefficient. The distributed phase rf coil is a fixed frequency coil, and also tends to generate undesirable electric fields.
More recently, a much simpler coil has been designed in which the two active sections of the coil are connected by two sections of transmission line to form a self-resonant loop or ringresonator. This arrangement overcomes most of the disadvantages of the previous coil designs and provides high frequency rf waves over a large volume. In order to tune this coil, however, a tuning capacitor must be provided at the center point of each section of transmission line. The use of two tuning capacitors is problematic, however, because the capacitors should be kept equal for optimum operation.
It would be advantageous to provide a still simpler rf coil design which would be more simply tuned. It would further be advantageous to provide such a coil capable of generating a circularly polarized wave.